JPH0370924B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to improving the transient characteristics of a power amplifier when the power switch is turned on, and in particular to a circuit that eliminates the hum sound that occurs during the muting period when the power is turned on. Concerning configuration.

[Conventional technology]

Conventionally, this type of low-frequency amplifier, such as a stereo, includes an input amplification circuit composed of a differential amplifier and an output circuit that amplifies the power of the output. A large transient current may flow into the speaker, causing abnormal noise (hereinafter referred to as "shock noise"). This shot sound is not only unpleasant to listen to, but can also sometimes damage speakers. Furthermore, since the input amplifier circuit placed before the power amplifier as an output circuit also generates a transient signal when the power is turned on, the transient signal of this preamplifier is amplified by the power amplifier, causing a large transient current to flow through the speaker, causing a shock. This will cause sound. Conventionally, in order to remove this kind of transient sound, it has been proposed to add a circuit that mutes the output for a period until the bias potential of the entire low frequency amplifier stabilizes. However, during this muting operation, there was a problem in that power supply hum noise was induced in the output.

Next, the problem of this power supply hum being output will be explained in more detail.

FIG. 1 specifically shows an embodiment of the present invention, in which transistors Q ₁₈ to Q ₂₀ , resistors R ₂₂ ,
A conventional configuration without R ₂₅ , R ₂₆ , R ₂₈ and constant current source I will be described. The input signal applied to input terminal B is input to the input amplifier circuit, that is, transistor _Q1 and
It is amplified by the differential amplifier configured by Q ₂ , phase inverted amplified by the level shift circuit of transistor Q ₄ , and further amplified by the transistor Q 4.
Amplified by Q ₁₄ and Q ₁₇ and connected to transistor Q ₁₂
The signal is transmitted to a class B push-pull output circuit composed of transistors Q ₁₃ and transistors Q ₁₅ and Q ₁₆ to drive the speaker S. Negative feedback is applied to this output signal to the base of input differential amplifier Q ₂ through a negative feedback circuit consisting of resistors R ₂₁ and R ₂₀ and capacitor C ₃ that set the voltage gain of this power amplifier. It works to amplify the power.

Next, the transient characteristics when the power is turned on will be explained.

Even if the power switch is turned on and the power supply voltages +V _CC and -V _EE are applied to terminals G and E, the capacitor C ₂ connected to terminal A is at 0 potential when the power is turned on, so at this point the transistor Q ₈ , Q ₉ , Q ₁₀ ,
Q ₁₁ , Q ₆ , Q ₇ , and Q ₅ are all in a non-conducting state. Therefore, the circuit shown in FIG. 1 does not operate as a power amplifier immediately after power is turned on.

Eventually, the capacitor C ₂ is charged from the resistor R ₇ and the voltage at the terminal A increases, and the transistors Q ₈ , Q ₆ ,
When the voltage exceeds the threshold voltage of Q ₇ (approximately 3V _BE , where V _BE is the forward voltage between the base and emitter of the transistor), transistor Q ₇ operates, then transistor Q ₅ operates, and transistors Q ₁ and Q ₂ The input differential amplifier consisting of starts operation. When transistor Q ₂ operates, transistors Q ₄ and Q ₁₄ , Q ₁₅
also works. When the potential at the terminal further increases and becomes equal to or higher than the threshold voltage (approximately 4V _BE ) of the circuit consisting of transistors Q ₈ , Q ₉ and diodes D ₆ , D ₇ , transistor Q ₉ operates, and transistors Q ₁₁ , Q ₁₂ , _Q13 ,
Q ₁₅ and Q ₁₆ start operating. The shock noise at the speaker terminal caused by the starting current when the upper and lower output transistors Q ₁₃ and Q ₁₆ start operating is made gentle by the time constant of the resistor R ₇ and capacitor C ₂ , so the shock noise generated at the speaker S can be reduced. . That is, until the charging voltage of the capacitor C ₂ exceeds the threshold voltage of the transistor Q ₈ , the output of the output terminal F is muted.
When the charging voltage exceeds the threshold voltage of the transistor _Q8 , the output at the output terminal F rises slowly, so that no shock noise is produced in the speaker S.

[Problem that the invention seeks to solve]

However, in the above-described mutating circuit of the conventional power amplifier, the base of the transistor _Q12 is in a floating state during the muting period from when the power is turned on until the output transistor operates, so a small leakage current flows between the power supply terminal G and the transistor _Q12. , in which case the leakage current flows through the diode D ₇ , the resistor R ₁₉ or the transistor Q ₁₂ to the speaker S
flows to This leakage current changes in response to the ripples superimposed on the power supply, so there is a drawback that the changes can be heard as a hum sound from the speaker.

Especially in recent years, high voltage monolithic power
When ICs are commercialized and used in high-output power amplifiers, many elements are integrated into a single chip, which requires a technology to prevent the leakage current failure described above. However, since the leakage current significantly increases as the temperature increases, screening at high temperatures is required, which is disadvantageous in that it becomes expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide a power amplifier equipped with a circuit for eliminating hum generated when a muting circuit operates when power is turned on.

[Means for solving problems]

According to the present invention, in a power amplifier that includes at least an input amplifier circuit and an output circuit, and in which the bias current supplied to the input amplifier circuit is supplied faster than the bias current supplied to the output circuit when the power is turned on, the output circuit A power amplifier is obtained having a circuit that bypasses the input end of the power amplifier for a predetermined period of time after power is turned on.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which transistors Q ₁₈ , Q ₁₉ ,
Q ₂₀ , resistors R ₂₂ , R ₂₅ , R ₂₆ , R ₂₈ and constant current source I
is added. Now, when the power is turned on, current is supplied to the base of the transistor _Q18 by the constant current source I, so the transistor _Q18 becomes conductive. As a result, the base of transistor Q ₁₂ is bypassed to the ground potential of terminal D, so even if leakage current as described above occurs at the bases of transistor Q ₁₂ and transistor Q ₁₅ , the output from output terminal F to the speaker No current flows due to leakage current. Therefore, no hum sound is generated in the speaker S.

Eventually, as described above, capacitor C ₂ is charged, transistors Q ₆ , Q ₇ , Q ₅ operate, and the input amplifier composed of transistors Q ₁ , Q _{2 ,} etc. is operated. Next, when transistors Q ₉ , Q ₁₀ , Q ₁₁ , and Q ₂₁ operate, transistor Q ₂₀ becomes conductive and transistor Q ₁₈ is turned off, current is supplied to output transistors Q ₁₂ and Q ₁₃ , and the potential of output terminal F increases. rises and starts supplying the amplified output to the speaker S.

〔Effect of the invention〕

As explained above, the present invention has the effect of eliminating the hum generated in the output speaker due to leakage current by bypassing the input terminal of the output circuit to GND during operation of the muting circuit.

In the present invention, we have mainly discussed the case where leakage current occurs at the base of transistor _Q12 , but this invention also applies to leakage current between the base of transistor _Q15 and the E terminal and between the base of transistor _Q16 and the G terminal. It is obvious that the circuit of the invention can be applied.

[Brief explanation of drawings]

FIG. 1 is a circuit connection diagram showing one embodiment of the present invention. R ₁ , R ₂ , ... R ₂₇ ... Resistance, Q ₁ , Q ₂ , ...,
Q ₂₀ ... Transistor, D ₁ , D ₂ , ... D ₁₀ ... Diode, C ₁ , C ₂ , ... C ₄ ... Capacitor, S
...Speaker, G...Positive power supply terminal, E...Negative power supply terminal, D...GND terminal, C...Negative feedback terminal,
B...Input terminal.

Claims (1)

[Claims] 1 a first output transistor section of one conductivity type connected between a first power supply terminal and an output terminal;
a second output transistor section of a reverse conduction type connected between a power supply terminal of the first transistor and the output terminal;
a first current source transistor having a collector-emitter path coupled between a terminal corresponding to the base of the output transistor section and the first power supply terminal; a terminal corresponding to the bases of the first and second output transistor sections; means for coupling, an input amplifier receiving an input signal, a second current source transistor serving as a current source for the input amplifier;
means for supplying a terminal corresponding to the base of the output transistor portion of the output transistor section; means for generating a time constant voltage that changes with a predetermined time constant in response to application of a power supply voltage between the first and second power supply terminals; means for converting the time constant voltage into a current and generating a first time constant current by being activated when the time constant voltage reaches a first voltage value, the time constant voltage being greater than the first voltage value; means for converting the time constant voltage into a current to generate a second time constant current when activated when a large second voltage value is reached; driving the second current source transistor with the first time constant current; and means for driving the first current source transistor with the second time constant current, thereby preventing a sudden change in the potential of the output terminal based on application of the power supply voltage. And,
The terminal is coupled to a terminal corresponding to the base of the first output transistor section, the terminal corresponding to the base is bypassed to the reference potential in response to application of the power supply voltage, and the terminal corresponding to the base is bypassed to the reference potential in response to the generation of the second time constant current. A power amplifier further comprising means for decoupling from a reference potential.